Test: If you get a lot of to-device msgs all at once, ensure they are processed in-order

[andybalaam]

See element-hq/element-web#25723

[kegsay]

This is tougher to do, as we really want the test itself to be a client so it can interleave misordered room keys around. It would require us to either use libolm (which is a hard dep on Complement today) or vodozemac. It feels wrong to be using vodozemac to test vodozemac though...

[kegsay]

One alternative would be to:

• block /sync
• Spin up a lot of clients (e.g 50) which will join the same room and send 5 msgs (causing to-device msgs to be sent), especially with a low rotation period (1 msg), this will cause 5x50=250 to-device msgs
• unblock /sync
• the next /sync will have hundreds of to-device msgs
• ensure all messages can be decrypted.

Given the original issue is tracking "skipped keys" which are per-device, it would be better to decrease the number of clients and increase the number of messages sent.